1. Field of the Invention
The present invention relates to a method of making a flexible sheet heater for use in waist pads, waist bandages, and clothes and more particularly, to a method of making a flexible sheet heater by using an electrically conductive fabric.
2. Description of the Related Art
Flexible, electrically conductive sheet heaters are intensively used in clothes, kneepads, gloves, shoe pads, ear covers, waist pads, and etc. to keep the user warm.
A conventional flexible, electrically conductive sheet heater is known comprising two flexible heat-resistant insulative sheets, a metal loop formed of a thin metal sheet member by chemical etching or stamping and sandwiched in between the two flexible heat-resistant insulative sheets, and two electrical terminals respectively connected to the two distal ends of the metal loop and extending out of the two flexible heat-resistant insulative sheets for enabling an electric current to be transmitted through the metal loop to generate heat. There is known another conventional structure of flexible, electrically conductive sheet heater, which comprises a first flexible heat-resistant insulative sheet, a carbon loop printed on the first flexible heat-resistant insulative sheet, a second flexible heat-resistant insulative sheet bonded to the first flexible heat-resistant insulative sheet to have the carbon loop be sandwiched in between the two flexible heat-resistant insulative sheets, and two electrical terminals respectively connected to the two distal ends of the carbon loop and extending out of the two flexible heat-resistant insulative sheets.
The aforesaid two different electrically conductive sheet heaters have light and thin characteristics and can be slightly curved; however, they cannot be folded up. Because the aforesaid two different electrically conductive heater systems are not foldable, their application is limited.
There is also known a flexible, electrically conductive sheet heater, which has an electrically conductive fabric, for example a carbon fiber fabric, sandwiched in between two flexible heat-resistant insulative coverings and provided with a power input structure. This design of flexible, electrically conductive sheet heaters has light and thin characteristics and is foldable for different applications. Exemplars of these electrically conductive fabric-based heater designs are seen in Taiwan Patent Publication No. 374,539, U.S. Pat. No. 6,172,344, and U.S. Pat. No. 6,483,087.
According to Taiwan Patent Publication No. 374,539, the heater comprises an electrically conductive rectangular carbon fiber fabric, two elongated conductive copper strips respectively affixed to two sides of the carbon fiber fabric, two electric wires respectively connected to the copper strips, and two plastic cover films respectively covered on the top and bottom sides of the carbon fiber fabric and the copper strips. According to this design, the conductivity of the copper strips is superior to the carbon fiber fabric, resulting in a high contact resistance between the two copper strips and the carbon fiber fabric. Further, because the surface of the carbon fiber fabric is not a smooth surface such that the whole surface of each copper strip is not fully kept in contact with the carbon fiber fabric, an excessive high temperature may be produced between the copper strips and the carbon fiber fabric. Taiwan Patent Publication No. 374,539 does no provide any measures to eliminate the aforesaid problems.
U.S. Pat. No. 6,172,344 discloses continuous and batch-based conductive element fabrication methods by sandwiching a carbonized fabric with electrical terminals between layers of plastic insulating material. However, U.S. Pat. No. 6,172,344 does not teach the way of keeping the carbonized fabric between the two layers of plastic insulating material in a smooth manner.
U.S. Pat. No. 6,483,087 discloses a process of making a heater by: combining a layer of electrically conductive fabric with two metal foil bus bars, securing the bus bars to the conductive fabric, drawing the conductive fabric layer containing bus bars between two layers of thermoplastic film forming a sandwich structure, feeding the sandwich structure through a pinch roller preheated at a predetermined temperature and thickness to cause gelling of the thermoplastic layers, and consolidating the conductive fiber layer to form a single sheet heater. According to this method, it is difficult to cut the layer of electrically conductive fabric into the desired curved shape. When cutting the layer of electrically conductive fabric, the carbonized fiber structure of the layer of electrically conductive fabric tends to be stretched and damaged, and the layer of electrically conductive fabric may be deformed easily when pulled toward the layers of thermoplastic film.